Rs485 loading problem

This sounds like it should be easy to diagnose. You say the signals on the RS-485 bus look OK, so check the next point. Keep following the signal until you find it not right somewhere.

The next logical point to test after the RS-485 bus would be when it gets converted back to a single-ended signal that presumably gets presented to UART receive pin of the device. Does that look right? If not, there is a problem with the RS-485 receiver circuitry. If so, then either the device is not working or you have a protocol problem.

In general, it's good to keep dividing up the suspect parts of the system to isolate what is not working, or to test them in isolation. For example, you could either try to tap the UART line into the device and see if you get what is expected by looking at the bytes on a PC with a low level program, or you could try a program on the PC to inject what you think is the right sequence of bytes and see if the device responds as you expect.

Chances are that if the UART signal at the device looks OK (including the obvious issue of baud rate), then you have a protocol problem. In other words, somehow you're not sending the right data to the device. Early on in bringing up a system like this, I often use a low level test program on the PC that sends and receives individual bytes. I can type byte values in decimal, hex, and as ASCII character, and it displays everything it receives likewise. This can be very useful for verifying that you really do understand the protocol the device expects.

Do some of these experiments and tell us what you find. It would also help to provide some information on this device you are trying to talk to from the micro.

Martin,

I'm by no means an expert at RS485, but I have run into some of the same problems you've had with independent power supplies on multi-drop RS485 networks.

Most frequently, in spite of claiming fail-safe operation, they don't play nice with ground loops/DC offsets. For me, in the past, the most direct route is to use fail-safe biasing on the two lines. E.g. .

By the way: Most of my learning has come from others here as well as some pretty good application notes from Texas Instruments (PDF), Maxim (HTML), and Analog Devices (PDF).

Hope to hear if this gets you in business.

Josh